Calcification is the process whereby the mineral calcium builds up in tissue, causing it to harden. Calcium that enters the body is normally deposited in bones and teeth (i.e. mineralized tissues). Ossified (hardened) tissues such as bone and teeth are composed of calcium phosphate and calcium carbonate deposits. In particular, the bulk of the mineral phase of bones and teeth is a poorly crystalline carbonate-rich analogue of the naturally occurring mineral hydroxyapaptite. When there is an imbalance in the body, calcium and other minerals can be deposited in other parts of the body such as arteries, aortic valves, kidneys, lungs, breast, brain, cartilage and tendons. Such deposits can disrupt the normal function of these tissues.
The calcification of healthy mineralized tissues results in ordered structures such as the hydroxyapatite rods found within dental enamel and dentin. However, in diseased states such as upon the attack of acid by acid-forming bacteria, the ordered structure is disturbed creating porous regions and amorphous areas.
Dental caries (i.e. dental cavities or dental decay) is a common oral disease that many people have experienced at some point in their life. With the introduction of fluoride in drinking water and toothpastes, there has been a decline in the prevalence of dental caries in Western countries.1 However, a recent report states that caries still remains a large problem in specific population groups (e.g. minority children, the economically underprivileged, older people, the chronically ill and institutionalized persons).2 Furthermore, the patterns of caries development are changing, to those with smaller lesion sizes and slower progression rates, making caries more difficult to detect with existing conventional techniques.1 This is further complicated when caries develop at locations which are not clinically visible (e.g. between adjacent teeth). Current diagnostic methods involve subjective clinical criteria (colour, “softness”, resistance to removal) and the use of tools such as the dental explorer and dental radiographs. Such methods may not be reliable for detecting interproximal lesions (those between adjacent teeth) because these carious lesions are not clinically visible. In addition, these clinical methods do not adequately detect changes in caries development and do not possess the sensitivity, specificity nor ability to account for the dynamic process of demineralization-remineralization.1,2 Therefore, more refined diagnostic tools are required to identify early non-cavitated carious lesions and to monitor their activity as well as severity. Early caries detection can potentially increase the implementation of conservative treatment methods centred on tooth preservation rather than restoration. Conservative methods include non-surgical interventions such as fluoride to promote remineralization, antimicrobials to arrest caries activity and sealants to prevent dental caries. New diagnostic techniques will enable the clinician to monitor patients for further lesion demineralization or remineralization, to evaluate the effectiveness of treatment strategies, as well as to encourage patient compliance in following suggested preventive measures. Several methods addressing the need for better early dental caries diagnostic tools with improved sensitivity and specificity have been investigated and recently reviewed.3-6 Among the methods are direct digital radiography (DDR), digital imaging fibre-optic trans-illumination (DIFOTI), electroconductivity measurements (ECM), quantitative light-induced fluorescence (QLF) and laser fluorescence.3 DDR has the capability to optimize diagnostic imaging operation by eliminating the need for film processing as well as to reduce the potential patient radiation dose. Although the technique uses a lower radiation dose, the application of the method is still limited by the ionizing radiation required to obtain an image. Like conventional radiographs, the technique provides little value in the detection of initial enamel lesions.3 DIFOTI, marketed by Electro-Optical Sciences, involves a trans-illumination technique through the use of a fibre-optic device to shine light on the tooth. The image captured represents the scattered and absorbed light as it traversed the tooth. Although the technique can detect incipient caries, the method is subjective, relying on the expertise of the examiner with a high level of intra- and inter-examiner variability.3,7 
Another technique, ECM is based upon the observation that sound surfaces have limited or no conductivity whereas surfaces with caries will have measurable conductivity that will increase with increasing demineralization. The drawback of the technique is its lack of specificity, which limits its usefulness in deciding whether or not to treat a lesion operatively. The low specificity increases the chance of false-positive results which translate into possibly unnecessary invasive treatment.5 Perhaps the most advanced of the technologies available is that based on fluorescence spectroscopy. With quantitative light-induced fluorescence, light at 488 nm is shone on a tooth and induces a natural fluorescence. Areas of demineralization appear as dark spots with mineral loss correlating with a relative loss of fluorescence radiance. With this method, the presence of bacterial plaque or saliva interferes with the usefulness of the technique.8 A related approach, laser fluorescence, uses light at 633 nm and the reflected fluorescence is measured. The light interacts with the bacterially produced porphyrins that leach into the decayed regions. Therefore fluorescence is suggestive of decay present. This is the basis for the commercially available DIAGNOdent device by KaVo. Recent clinical studies indicate that this latter technique can lead to false-positive results due to the presence of stains, deposits, calculus and organic material in the region of interest9 and is not suitable for detecting initial carious changes in enamel.10 Therefore, despite the potential of these various technologies, the methods are prone to subjectivity issues with high intra- and inter-examiner variability, false-positive results due to stains or organic deposits and the unsuitability for detecting initial enamel caries at interproximal sites. As a result of these shortcomings, more refined tools with improved sensitivity and specificity are needed for early detection of interproximal dental caries.